It is known that reduced coenzyme Q10 can be prepared by producing coenzyme Q10 in such conventional manner as synthesis, fermentation, or extraction from natural products, and concentrating a reduced coenzyme Q10-containing eluate fraction resulting from chromatography, and by the like method (JP-A-10-109933). On that occasion, as described in the above-cited publication, the chromatographic concentration may be carried out after reduction of oxidized coenzyme Q10 contained in the reduced coenzyme Q10 with a conventional reducing agent such as sodium borohydride or sodium dithionite (sodium hydrosulfite), or reduced coenzyme Q10 may be prepared by reacting an existing highly pure grade of coenzyme Q10 with the reducing agent mentioned above.
However, the thus-obtained reduced coenzyme Q10 is not always in a highly pure state but is often in a low-purity crystalline, oily or semisolid form, which contains impurities such as oxidized coenzyme Q10, for instance.
As a result of intensive investigations, the present inventors have established several methods of obtaining high-quality reduced coenzyme Q10 and applied for patent (Japanese Patent Application Nos. 2002-114854, 2002-114871, 2002-114872, 2002-114873, 2002-114874, 2002-114875, 2002-114876, 2002-114877, 2002-114878, and 2002-114879).
However, reduced coenzyme Q10 is readily oxidized to oxidized coenzyme Q10 by molecular oxygen and, even when high-quality reduced coenzyme Q10 is produced by such methods as those disclosed in the above-cited patent applications, it is still a big problem how to stabilize reduced coenzyme Q10 in processing it into foods, functional nutritive foods, specific health foods, nutritional supplements, nutrients, animal drugs, drinks, feeds, cosmetics, medicines, remedies, preventive drugs, etc., or raw materials or compositions therefor and/or storing such processed products after preparation. In the above-mentioned processing or storage, it is very difficult to completely eliminate or shield against oxygen, and residual oxygen or contaminant oxygen exerts a great adverse influence upon warming in processing or during long-term storage, in particular. The above-mentioned oxidation is directly connected with such a quality problem as the formation of oxidized coenzyme Q10 as a byproduct.
Thus, it is a very important problem to stabilize (protect against oxidation) reduced coenzyme Q10. Since, however, reduced coenzyme Q10 has not been commercialized up to the present, there have been few studies done on the method and composition for stably maintain reduced coenzyme Q10. In the only example published (WO 01/52822), there are described a composition coexisting a reducing agent and preparation method thereof. Disclosed in that document are:                1) A composition which comprises reduced coenzyme Q10, an effective amount of a reducing agent in preventing oxidation of reduced coenzyme Q10 to oxidized coenzyme Q10, and a surfactant or a vegetable oil or a mixture of these in an amount effective in dissolving the reduced coenzyme Q10 and the reducing agent, optionally together with a solvent;        2) A composition for oral administration obtained by forming the above composition into a gelatin capsule or a tablet; and, further,        3) A method of preparing the above composition which contains reduced coenzyme Q10 prepared in situ by using oxidized coenzyme Q10 and a reducing agent.        
However, the above publication WO 01/52822 has no detailed description of the quality of reduced coenzyme Q10 contained in the composition, the stabilizing effect, or the like. Moreover, the above composition and the above method of preparation are very complicated and troublesome since the composition has to play a plurality of roles (namely, a first role as the field of reaction for reducing oxidized coenzyme Q10 to reduced coenzyme Q10, and a second role in stably maintaining reduced coenzyme Q10).
Furthermore, it is to be noted that the reaction mixture itself is directly used as such in the above composition or method of preparation and, therefore, it is hard to say that the composition is always safe. More specifically, while an ascorbic acids is used as the reducing agent in reducing oxidized coenzyme Q10 to reduced coenzyme Q10, the composition is contaminated with significant amounts of the corresponding dehydroascorbic acid, 2,3-diketoglucuronic acid, threonic acid, oxalic acid and the like as a result of oxidation of such ascorbic acids. Unlike ascorbic acids, dehydroascorbic acids and oxalic acid resulting from decomposition are highly harmful. For example, they reportedly increase the lipid peroxide level and decrease the antioxidant substance level in the liver and kidney, and increase the oxalic acid level in the kidney and there is a fear of adverse effects, for example, decreases in resistance to oxidative stress, symptom of urolithiasis and the like (Nutrition Research, vol. 13, pp. 667-676, 1993).
As for the composition containing reduced coenzyme Q10, the above-cited JP-A-10-109933 discloses a composition comprising 0.3 g of coenzyme Q10 (oxidized form: reduced form=5:95) and 6.0 ml (5.45 g) of olive oil (reduced coenzyme Q10 content in the composition=4.96% by weight) and a composition comprising 20 parts by weight of coenzyme Q10 (oxidized form: reduced form=15:85), 15 parts by weight of vitamin E and 350 parts by weight of soybean oil (reduced coenzyme Q10 content in the composition=4.42% by weight; vitamin E content based on the system excluding coenzyme Q10: 4.11% by weight).
However, in the above publication, there is no description at all of the stability of reduced coenzyme Q10, for example and, as a result of investigations made by the present inventors, it was found that the above compositions are not always preferred as compositions for stably maintaining reduced coenzyme Q10.